Analog Input

The term pseudodifferential refers to the 50 Ω resistance between the AI- terminal and chassis ground.

If the signal source is floating, use the pseudodifferential configuration. The pseudodifferential configuration provides a ground reference between the floating source and the DSA device by connecting a 50 Ω resistor (depending on the DSA device) from the negative input to ground. Without this, the floating source can drift outside of the common-mode range of the DSA device being used.

If the signal source is grounded or ground referenced, both the pseudodifferential and differential input configurations are acceptable. However, the differential input configuration is preferred, since using the pseudodifferential input configuration on a grounded signal source creates more than one ground-reference point. This condition may allow ground loop currents, which can introduce errors or noise into the measurement. The 50 Ω resistor between the negative input and ground is usually sufficient to reduce these errors to negligible levels, but results can vary depending on your system setup.

Configure the channels based on the signal source reference or DUT configuration. Refer to the following table to determine how to configure the channel.

The PXIe-4468 supports AC and DC coupling. Use the AI.Coupling property to configure the device or module for either AC or DC coupling.

• Select DC-coupling if the signal source has only small amounts of offset voltage or if the DC content of the acquired signal is important. When you select DC coupling, any DC offset present in the source signal is passed to the ADC.
• Select AC coupling if the signal source has a significant amount of unwanted offset so it can take full advantage of the input dynamic range. Selecting AC coupling enables a highpass resistor-capacitor (RC) filter into the signal conditioning path.
  • The filter time constant is 200 ms. The highpass RC filter settles to 2% accuracy in 1 s in response to a step input. It takes 3.128 s to settle to 24-bit accuracy in response to a step input. The settling time is somewhat dependent on the DUT impedance as well.

Using AC coupling results in an attenuation of the low-frequency response of the AI circuitry. Refer to the specifications for your product for information about the cut-off frequency.

A sensor or transducer is a device that outputs an electrical signal in response to a measured physical phenomenon such as pressure or temperature. The most common sensors for use with DSA devices include microphones for measuring sound pressure and accelerometers for measuring linear acceleration or vibration.

When the signal voltage exceeds the input range, distortion caused by a clipped or overranged waveform can occur. The PXIe-4468 includes overload detection in both the analog domain (predigitization) and digital domain (postdigitization).

Use the OverloadedChansExist and OverloadedChans properties to access the overload detection feature.

An analog overrange can occur independently from a digital overrange, and vice versa. For example, an IEPE accelerometer might have a resonant frequency that, when stimulated, can produce an overrange in the analog signal. However, because the ADC delta-sigma technology uses very sharp anti-aliasing filters, the overrange is not passed into the digitized signal.

Conversely, a sharp transient on the analog side might not overrange, but the step response of the delta-sigma anti-aliasing filters might produce clipping in the digital data. The PXIe-4468 analog overload detection circuitry detects a clipped or overloaded condition. You can programmatically poll the overload detection circuitry on a per channel basis to monitor for an overload condition. If an overload is detected, consider any data acquired at that time corrupt.

DSA devices perform digital overload detection as a percentage of the range. The overload detection occurs before the device applies gain and offset corrections. Detecting the overload before the gain and offset corrections catches an overflow condition in the delta-sigma modulator or ADC filter.

For instance, on the PXIe-4468, the analog overload point for the 0 dB gain range is approximately 10.7 8 pk. This is the voltage at which the front-end circuitry begins showing signs of saturation.

The PXIe-4468 analog-to-digital converters (ADCs) use the Delta-Sigma modulation conversion method. This approach involves oversampling the input signal and then decimating and filtering the resulting data to achieve the desired sample rate.

The filter delay is the time required for data to propagate through a converter. All DSA device channels have filter delays due to the presence of filter circuitry on both input and output channels. The digital filter delay is compensated to 0 ns by default. You can adjust the filter delay in NI-DAQmx.